Well, they say a tire loses a pound per month, sure enough, when I needed to put on my spare after 30 months it was 30# down from the recommended 60#. Check your spare, and remember front to rear, spare to front ONLY (LSD).

Check your tire pressure gauge w/ a shop or a trusted reference...I've been over-inflated w/o knowing it till a recent tiire repair showed a difference between the 'new' and the other three!